The invention relates to a frequency response equalization circuit particularly suitable for magnetically recorded and reproduced signals where phase linearity over a wide frequency range is required.
Signals subjected to magnetic recording and reproducing process are generally distorted due to a non-constant amplitude response and non-linear phase response inherent to the process. Consequently, if an accurate replica of a magnetically recorded signal is to be reproduced, it is necessary to compensate for the signal distortion. A basic requirement for an ideal magnetic recording/reproducing channel is the ability to transmit signals of all frequencies without introducing amplitude changes dependent on frequency. A specific limitation of the magnetic recording/reproducing process is that signals of different frequencies are transmitted through the channel with different amounts of time delay due to the differentiating action of the reproduce head introducing a substantially 90.degree. phase lead. Thus, when transmitting a complex waveform consisting of a number of sine wave components of different amplitudes and frequencies, the sine wave components are affected differently, thereby introducing unwanted signal distortion. To obtain an accurate reproduction of the originally recorded complex waveform, the various waveform components must be transmitted with an equal amount of time delay for all signal frequencies otherwise the resulting reproduced waveform would be seriously distorted. To achieve a constant time delay that is, of the same magnitude over the entire bandwidth of a recording/reproducing channel, the phase response of transmitted signals having different frequencies must change linearly with frequency.
There is known a number of prior art equalizers having cascaded passive and/or active elements, whose combined frequency responses yield an equalizing characteristic compensating for non-linearities causing signal distortions in a specific recording/reproducing channel. For example, for signal equalization over both low and high frequency range of a recording/reproducing channel, known prior art equalizers have several consecutive stages, such as high frequency passes and low frequency passes, usually followed by phase equalizers. These cascaded equalizers have a rather complex design.
In digital recording on a polarizable storage medium, such as magnetic or dielectric, where two levels of signals of opposite polarities are utilized to record signal bits, phase linearity within the entire bandwidth of the recording/reproducing channel is essential to permit reproduction of the originally recorded square waves with minimum overshoot and amplitude degradation. When utilizing a high bit density self-clocking recording system the need for both amplitude and phase equalization is even more pronounced. Due to the non-linear frequency response of the recording/reproducing channel, a recorded square wave signal when played back directly from a polarizable storage medium could be seriously attenuated and distorted to such extent that it would neither provide sufficient information about the originally recorded signal nor provide distinct zero crossovers necessary for an accurate self-clocking signal. Thus, amplitude as well as phase equalization is necessary to restore the reproduced signal to its original form.